More or less uninterrupted uptime of different critical loads in electrically powered systems is becoming increasingly important. Therefore systems of today often have adjoining power back-up arrangements to provide uninterrupted power to such critical load if the power from the utility line to the system is interrupted for whatever reason.
A power back-up arrangement normally comprises a battery for storing and providing back-up power. If the voltage to the load decreases below a threshold value, the stored charge in the battery will be provided to the critical load. The power of the battery will continue to be provided to the load until the power from the utility line has returned alternatively until the battery is drained of power.
Thus, in order to investigate the state of a battery in a power back-up arrangement, different tests on the battery may be needed. One such test is a capacity test. That is testing the battery's stored electric charge. The nominal specification of a battery may often only be valid at preset operational temperatures which normally are lower or higher than the actual operating temperature of the battery. This makes the nominal specification unreliable and unpractical in order to determine the capacity from the battery specification. Another uncertainty is the normal decrease in capacity of a battery affected by charge and discharge, i.e. capacity decrease in relation to the number of battery cycles the battery has been subject to as well as the decrease in capacity due to aging of the battery.
Several methods of predicting and measuring the capacity and status of a battery in a power back-up system exist. Measuring by discharge may be adding a risk of decreased capacity of the back-up power. Thus, different ways to of testing the status and capacity of a power back-up without discharge has been proposed. One example of such a method is performed by measuring impedance and/or conductance of the battery. Based on that measurement, the capacity and status of the battery can be theoretically determined. Another example is to discharge the battery to a certain level of its capacity and, based on such partly discharge test, determine the resulting capacity of the battery by extrapolation. These testing methods are however rather inaccurate and the remaining back-up power capacity to handle a mains interruption in connection to the test is sometimes unknown.
FIG. 1 is a schematic block diagram showing a power back-up system with a battery according to the prior art. The power back-up system is connected to a load 101 in order to provide uninterrupted power supply in the event of utility mains interruption.
In FIG. 1, the utility mains 109,109′ are connected to a power supply 100 which is adapted to charge the battery strings 103,103′ and to provide power to the load 101. The battery strings103,103′ are arranged to be in connection with the power supply 100 and load 101 in order to receive charge from the power supply 100 and to be capable of providing power to the load 101 if the utility mains 109,109′ is interrupted.
FIG. 2 shows a similar arrangement as of FIG. 1 but with an additional arrangement for testing capacity by battery partial discharge. In FIG. 2, a switch 105 is arranged at the utility mains 109,109′ and adapted to disconnect the utility mains 109;109′ from the power supply 100. The arrangement further comprises battery test equipment 102 which is operatively connected to the switch 105. The battery test equipment 102 can also be operatively connected to the power supply 100. The battery test equipment 102 is adapted to interrupt the mains 109,109′ by sending a switch signal to the switch 105. Such interruption is normally done manually by testing personnel. An alternative to interrupting the mains may enable the battery test equipment 102 to instruct the power supply 100 to reduce the output voltage below a minimal battery discharge level in order to ensure that the power provided to the load 101 is taken from the battery 103,103′. The battery test equipment 102 is further connected to a current sensor 104 and a voltage sensor 107 in order to monitor the discharge test. The discharge test arrangement illustrated in FIG. 2 normally requires manual intervention and manual supervision in order to perform a sufficient partial discharge without reaching critical levels.